Essential Cell Biology 5th edition

386 CHAPTER 11 Membrane Structure
neutrophil
specific
oligosaccharide
LECTINS RECOGNIZE
CARBOHYDRATES ON
NEUTROPHIL
NEUTROPHIL ROLLS
ALONG BLOOD
VESSEL WALL
BLOOD
ADDITIONAL INTERACTIONS
ALLOW NEUTROPHIL TO
MIGRATE INTO INFECTED TISSUE
blood
vessel
lectin
endothelial cell
TISSUE
SITE OF INFECTION
Figure 11–37 The recognition of cellsurface
carbohydrates on neutrophils
allows these immune cells to begin
to migrate out of the blood and
into infected tissues. Specialized
transmembrane proteins (called lectins) are
made by the endothelial cells lining the
blood vessel in response to chemical signals
emanating from a site of infection. These
proteins recognize particular sugar groups
carried by glycolipids and glycoproteins on
the surface of neutrophils (a type of white
blood cell, also called a leukocyte)
circulating in the blood. The neutrophils
consequently stick to the endothelial
cells that line the blood vessel wall. This
association is not very strong, but it leads
to another, much stronger protein–protein
interaction (not shown) that helps the
neutrophil slip between the endothelial
cells, so it can migrate out of the
bloodstream and into the tissue at the site
of infection (Movie 11.9).
interact with it. Specific oligosaccharides in the carbohydrate layer are
involved, for example, in the recognition of an egg by sperm (discussed
in Chapter 19). Similarly, in the early stages of a bacterial infection, carbohydrates
on the surface of white blood cells called neutrophils are
recognized by a lectin on the cells lining the blood vessels at the site of
infection; this recognition causes the neutrophils to adhere to the blood
vessel wall and then migrate from the bloodstream into the infected tissue,
where they help destroy the invading bacteria (Figure
ECB5 e11.37/11.37
11–37).
ESSENTIAL CONCEPTS
• Membranes enable cells to create barriers that confine particular
molecules to specific compartments. They consist of a continuous
double layer—a bilayer—of lipid molecules in which proteins are
embedded.
• The lipid bilayer provides the basic structure and barrier function of
all cell membranes.
• Membrane lipid molecules are amphipathic, having both hydrophobic
and hydrophilic regions. This property promotes their spontaneous
assembly into bilayers when placed in water, forming closed compartments
that reseal if torn.
• There are three major classes of membrane lipid molecules: phospholipids,
sterols, and glycolipids.
• The lipid bilayer is fluid, and individual lipid molecules are able to
diffuse within their own monolayer; they do not, however, spontaneously
flip from one monolayer to the other.
• The two monolayers of a cell membrane have different lipid compositions,
reflecting the different functions of the two faces of the
membrane.
• Cells that live at different temperatures maintain their membrane fluidity
by modifying the lipid composition of their membranes.
• Membrane proteins are responsible for most of the functions of cell
membranes, including the transport of small, water-soluble molecules
across the lipid bilayer.
• Transmembrane proteins extend across the lipid bilayer, usually as
one or more α helices but sometimes as a β sheet rolled into the form
of a barrel.
• Other membrane proteins do not extend across the lipid bilayer but
are attached to one or the other side of the membrane, either by
noncovalent association with other membrane proteins, by covalent
attachment of lipids, or by association of an exposed amphipathic
α helix with a single lipid monolayer.